In various applications in the graphic display field, a requirement exists to move characters about a video screen. A foremost aspect of such graphic display is that the movement of the characters appear smooth. Particularly demanding applications are found in the fields of media and public speaking, wherein teleprompters vertically scroll text to prompt the reader. As perceived by a listener, the reading must be natural, as if the speaker is talking without the aid of a teleprompter. It is generally known in the art that smoothly scrolling text improves readability of moving text, decreases eye fatigue and, generally makes the speech appear more natural. Of significance also are such considerations as independence of the speaker's memory capacity and control of the time of presentation, the latter being fully controlled by the teleprompter apparatus based on control of the speed of movement of text across the display screen.
In teleprompters in use today, the smoothness of the vertical movement is a function of the number of vertical scan lines. The addition of more scan lines decreases the size of each vertical step taken by the display character, thereby smoothing the movement. However, standard video displays in use today in television have fixed formats comprised of a fixed numbers of scan lines. Additionally, the cost of the video display increases as the number of scan lines are increased.
One practice in prior art is to simulate additional scan lines (fractional scan lines) at the leading edge of the character by creating an illusion that fractional scan lines exist. Such illusion is accomplished by alternately displaying two adjacent scan lines, one scan line being the leading edge of the character, the other being the background immediately in front of the character. The human eye averages the two scan lines together thereby creating the illusion of an fractional scan line. When this method is employed, due to the decreased display frequency of each scan line, the two adjacent scan lines appear to flicker. Taking this method one step further, multiple fractional scan lines can be created by varying the periodicity in which two adjacent scan lines are displayed. However, since the rate of display of one of the scan lines is further decreased, the amount of flickering increases.
Another observation that is noted as background to the subject invention is that teleprompting systems heretofore known and effecting smooth scrolling involve particularly designed and dedicated computer hardware and are computationally intensive. The art has not heretofore seen implementation of teleprompting with smooth scrolling by generally usable computational equipment, such as the conventional personal computer.